1. Field of the Invention
The present invention relates to a pellicle inspection apparatus and, more specifically, to a pellicle inspection apparatus suitable for inspecting a pellicle film on an EUV mask provided with a pellicle used in an EUVL (Extremely Ultraviolet Lithography) process.
2. Description of Related Art
Currently, ArF lithography is applied to a quantity production in a lithographic technology having a key role in miniaturization of semiconductors. In the ArF lithography, an ArF excimer laser having an exposure wavelength of 193 nm is used as an exposure source. An immersion technology for enhancing a resolution by filling a space between an object lens of an exposure apparatus and a wafer with water (referred to as “ArF immersion lithography”) is also started to be used in the quantity production. In addition, in order to achieve further miniaturization, technological development aiming at commercial viability of lithography having an exposure wavelength of 13.5 nm is widely performed as a process of manufacturing next generation semiconductor device. An EUV mask has a structure in which a reflective film for reflecting EUV light is formed on a substrate formed of a low-thermal expansion glass. The reflective film generally has a multilayer film structure having molybdenum films and silicon films layered alternately by a several tens of layers, and is capable of reflecting normal incident EUV light having a wavelength of 13.5 nm as much as approximately 65%. With formation of an absorber that absorbs the EUV light on the multilayer film, blanks are formed. However, a protective film (a film referred to as a buffer layer and a capping layer) is formed between the absorber and the multilayer film. In order to actually use in exposure, a pattern is formed on a photoresist, so that the EUV mask (referred to as a EUV mask blank because the pattern is not yet formed thereon) is completed.
In the ArF lithography of the related art, a photomask provided with a pellicle for preventing foreign substances such as particles in the atmospheric air from being adhered to a pattern surface of the photomask is used. In the photomask provided with a pellicle, the pattern surface of the photomask is surrounded by a transparent thin film referred to as a pellicle film. Even when particles having sizes not larger than a certain size are adhered to the pellicle film, the particles adhered to the pellicle film exist at positions displaced in a direction of an optical axis with respect to the pattern surface. Therefore, at the time of exposure, images of the particles projected on the wafer are out of focus and hence are not exposed onto the wafer. Therefore, even though the foreign substances such as particles are suspending in the atmospheric air, the images of the foreign substances are not projected on the wafer owing to the presence of the pellicle. The pellicle is very effective as a member to achieve accurate exposure.
In contrast, the EUV light having a wavelength of 13.5 nm does not pass through the pellicle film formed of a high-polymer material for photomasks at all. Therefore, in the related art, it is considered that the pellicle cannot be provided on the EUV mask. However, it is known that silicon films and zirconium films being extremely thin having a thickness of 100 nm or smaller, transmit the EUV light to some extent. A development of the pellicle for the EUV mask using these thin films is now being performed. In particular, a monocrystalline silicon film having a thickness on the order of 50 nm is capable of transmitting approximately 90% of the EUV light. Therefore, a development of the EUV pellicle using such a silicon film is now being performed (for example, see Japanese Unexamined Patent Application Publication No. 2009-116284).
In addition, using an Si/Mo/Nb film or a polycrystalline silicon film is also reported as another type of the pellicle film (for example, see Luigi Scaccabarozzi, et al. “Investigation of EUV pellicle feasibility,” SPIE Vol. 8679, 867904, 2013.). In the “Investigation of EUV pellicle feasibility”, a problem of wrinkles formed on the pellicle film is also reported. In the “Investigation of EUV pellicle feasibility”, as regards influence exerted on an imaging performance of the pellicle film, an extent of wrinkles formed on the pellicle film is expressed by using a ratio (A/Λ) between an amplitude A and a cycle Λ. In the “Investigation of EUV pellicle feasibility”, a value of A/Λ preferably does not exceed 200 mrad, and it is reported that an amount of displacement ΔCDU of the pattern becomes 0.1 or lower if the value of A/Λ does not exceed 200 mrad.
In the exposure process of the related art, even though the foreign substances are adhered to a surface of the pellicle film, the images of the foreign substances are not projected on the wafer. Therefore, the foreign substances inspection on the pellicle film has not been considered as an important matter. However, in the EUVL, when the foreign substances are adhered to the surface of the pellicle film, the EUV light is absorbed by the foreign substances. Therefore, a problem of errors which may occur on the pattern projected on the wafer is envisioned. Therefore, in the EUVL, conducting the foreign substance inspection on the pellicle film is strongly desired.
Examples of the effective method of inspecting foreign substances existing on the pellicle film include a laser scattering light system. In the laser scattering light system, the surface of the pellicle film is irradiated obliquely with an illuminating beam, and scattering light generated from the foreign substances is detected. As a unique problem of the EUV mask, a problem of formation of wrinkles and sagging on the pellicle film as described in the “Investigation of EUV pellicle feasibility” is pointed out. In other words, the monocrystalline silicon film is a hard thin film, and has a thickness on the order of 50 nm, which is extremely thin. If a strong tensile force is applied, the pellicle film may crack. Therefore, giving some leeway to the pellicle film is required when adhering the pellicle film on a pellicle frame. The pellicle film may be subjected to formation of wrinkles and sagging. The formation of winkles and sagging may occur in various forms. In contrast, when projecting a laser beam on the sagged pellicle film, if an amount of sagging exceeds locally a threshold value, a regular reflection light outgoing from the pellicle film enters an object lens. Therefore, a problem that an accurate foreign substance inspection cannot be performed may occur. In addition, the amount of sagging and an amount of wrinkles generating on the pellicle film are non-uniform. Depending on the form of the sagging, a problem that an amount of scattering light collected by the object lens varies may occur. In other words, depending on a direction or the form of the sagging formed on the pellicle film, a direction and an amount of scattering light outgoing from the foreign substances vary. Therefore, a problem that the intensity of the scattering light collected by the object lens varies may occur.
In addition, as a unique problem of the EUV mask, a problem of carbon contamination is pointed out. When the EUV mask is irradiated with EUV light, organic substances suspending in the vicinity of the pellicle film may be decomposed by the EUV light. Accordingly, carbide is accumulated on the surface of the pellicle film, and a thin film formed of the carbide is formed thereon. The film of the carbide formed by the carbon contamination has an effect of absorbing the EUV light. When the carbon contamination occurs, the EUV light is absorbed by the carbide film, and a problem that an accurate pattern drawing is not achieved may occur. Therefore, in the inspection of the EUV mask provided with a pellicle, capability of detecting not only the foreign substances such as particles but also contamination formed on the surface of the pellicle film in a thin film form is strongly required.
In addition, as a problem of the foreign substance inspection of the laser scattering light system, incident of inspection light onto the mask pattern through the pellicle film and generation of scattering light from the mask pattern may be exemplified. When the scattering light is generated from the mask pattern, the scattering light is received by the object lens. Consequently, sensitivity of defect detection is lowered, and a problem of generation of a pseudo defect occurs.
It is an object of the invention to realize a pellicle inspection apparatus in which problems unique to a pellicle inspection of an EUV mask is solved.
It is another object of the invention to realize a pellicle inspection apparatus capable of achieving an inspection with high degree of accuracy even though sagging or formation of wrinkles occurs on the EUV mask.
It is still another object of the invention to provide a pellicle inspection apparatus capable of detecting both foreign substances and contamination.